Method of estimating properties of steel product

ABSTRACT

A method of estimating the properties of a steel product, comprising completing a computation for determining metallurgical phenomena based on information concerning steel ingredients and production conditions in steps from casting to heat treatment to successively determine the state of a metallic structure and estimating the properties of a steel product from the final state of the metallic structure. More specifically, the method comprises the steps of: computing the temperature of a slab based on casting conditions and further completing a computation for the state of the metallic structure after casting based on information concerning the temperature of the slab and steel ingredients; completing a computation for the state of the metallic structure after rolling based on the state of the metallic structure before the rolling and the temperature of the steel product computed from rolling conditions; completing a computation for the state of the metallic structure after cooling based on the state of the metallic structure before the cooling and the temperature of the steel product computed from cooling conditions; and completing a computation for the properties of the steel product based on the final state of the metallic structure. In order to cope with the provision of additional production steps such as reheating, a homogeneous diffusion heat treatment, preliminary rolling, quenching, tempering and normalizing, etc., the method may further comprise the steps of completing a computation for the state of the metallic structure after applying the additional steps based on the state of the metallic structure before applying the additional steps and the conditions for the additional steps.

TECHNICAL FIELD

The present invention relates to a method of estimating the propertiesof a steel product which enables the structure and properties of thesteel product to be estimated without conducting a breaking test duringor after the production of the steel product.

BACKGROUND ART

For example, users of steel plates etc. often demand of the manufacturerthe attachment of the results of a material test concurrently with thedelivery of the product. In response to this demand, the manufacturerhas hitherto cut out a part of the product and conducted a test of thephysical properties (tensile strength, toughness, etc.) of the extractedsample.

In some cases, the properties of the steel product have been estimatedbased on production conditions such as steel ingredients and rollingtermination temperature. Since, however, this is usually done using aregression method and does not take metallurgical phenomenon intoconsideration, the scope of application is limited and disadvantageouslythis method cannot be applied to different production processes or steelplate thicknesses.

The above-described artificial measurement of properties requires a lotof time, which influences the shipment and delivery of the product.Further, at the present time, the properties thereof can be discernedonly in a finished product, and the development of a technique thatenables the properties of the steel product to be estimated beforeproduction thereof and, at the same time, provides production conditionscapable of providing a high quality goods consistently, is required inthe art.

Accordingly, an object of the present invention is to provide a methodof estimating the properties of a steel product that enables theproperties of the steel product to be automatically assessed based ongiven production conditions.

DISCLOSURE OF THE INVENTION

In the present invention, in order to attain the above-described object,a computation for metallurgical phenomena is computed based oninformation concerning steel ingredients and production conditionsduring the casting to heat treatment steps to successively determine thestate of a metallic structure, and the properties of a steel product isestimated from the final state of the metallic structure.

Specifically, the method of estimating the properties of a steel productaccording to the present invention comprises the steps of: computing thetemperature of a slab based on casting conditions and completing acomputation for the state of the metallic structure after casting basedon information concerning the temperature of the slab and the steelingredients; completing a computation for the state of the metallicstructure after rolling based on the state of the metallic structurebefore rolling and the temperature of the steel product computed fromrolling conditions; completing a computation for the state of themetallic structure after cooling based on the state of the metallicstructure before cooling and the temperature of the steel productcomputed from cooling conditions; and completing a computation for thequality of the steel product based on the final state of the metallicstructure.

In order to cope with the provision of additional production steps suchas reheating, a homogeneous diffusion heat treatment, preliminaryrolling, quenching, tempering and normalizing, etc., the method mayfurther comprise the step of completing a computation for the state ofthe metallic structure following the additional steps based on the stateof the metallic structure before the additional steps and the conditionsof the additional steps.

Further, instead of completing a computation for the state of themetallic structure after casting, the state of the metallic structureafter casting may be assumed and taken as an initial state.

According to the above-described means, the input of actual results ofthe production of a steel product or conditions set before theproduction of the steel product followed by a computation enables afraction of a phase constituting the structure as the key to the qualityof the steel product (tensile strength, toughness, etc.), the amount offormation, grain diameter and state of solid solution and precipitationof the phase at each temperature, etc. to be determined at any stage inthe production process, which enables the properties of the steelproduct to be estimated during production thereof. Further, it becomespossible to set production conditions that can assuredly realize thequality specifications of the steel product. Thus, as opposed to theconventional method, testing and measurement of the finished product canbe significantly reduced or eliminated.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block flow diagram of equipment showing the outline of asteel product production line to which the present invention is applied;

FIG. 2 is a diagram showing a construction and a computing flow of acasting model;

FIG. 3 is a diagram showing a construction and a computing flow of aheating model;

FIG. 4 is a diagram showing a construction and a computing flow of a hotrolling model;

FIG. 5 is a characteristic diagram showing a change in the dislocationdensity during rolling;

FIG. 6 is a diagram showing a construction and a computing flow of atransformation model;

FIG. 7 is a diagram showing a construction and a computing flow of astructure-property model;

FIG. 8 is a diagram showing a comparison of the measured value with thevalue calculated according to the method of the present invention withrespect to the yield strength;

FIG. 9 is a diagram showing a comparison of the measured value with thevalue calculated according to the method of the present invention withrespect to the tensile strength; and

FIG. 10 is a diagram showing a comparison of the measured value with thevalue calculated according to the method of the present invention withrespect to the ductile/brittle fracture transition temperature (vTrs) ina V Charpy impact test.

BEST MODE FOR CARRYING OUT THE INVENTION

An embodiment of the present invention will now be described taking theproduction of a steel plate as an example.

FIG. 1 is a block flow diagram of equipment showing the outline of asteel product production line to which the present invention is applied.

As shown in FIG. 1, the process is roughly classified into a steelmakingstep and a steel plate production step. The steelmaking step is dividedinto refining and casting (solidification). The present invention isdirected to steps after casting. Although there is no universal methodof classifying the steel plate production step, in the presentinvention, the steel plate production step is classified into a slab.treatment (a homogeneous diffusion heat treatment or preliminaryrolling), reheating, rolling, cooling and a heat treatment (quenching,tempering or normalizing).

The slab cast in the casting equipment (continuous casting or ingotmaking equipment) is carried to a plate mill which comprises a soakingpit for subjecting the slab to a homogeneous diffusion heat treatment, aheating furnace for heating the slab before rolling, a roughing mill forconducting rough rolling, a finishing mill for rolling the roughlyrolled steel plate to a necessary plate thickness, a hot leveler (HL)for correcting the warpage in the steel plate caused by the finishingmill, a cooling device for cooling the steel plate withdrawn from thehot leveler and a heat treatment device for heat-treating the steelplate. The heat treatment device comprises a heating furnace and acooling device for quenching.

Process computers (hereinafter referred to as "PROCOM") (PROCOM forsteel making blooming, PROCOM for heating, PROCOM for rolling, PROCOMfor cooling and PROCOM for a heat treatment) are connected to respectiveequipment and devices for the purpose of driving and controlling theequipment and devices. These PROCOM's are connected to a host computer(not shown) provided in a central control room, and the host computercontrols each PROCOM according to production planning. Further, amechanical test system is provided for a steel plate product for thepurpose of conducting a material test. The test results are sent to thecentral control room.

The method of estimating the properties of the steel product accordingto the present invention roughly comprises 11 fundamental models, thatis, the following models corresponding to respective production steps ofthe steel product

a casting model (a casting step),

a homogeneous diffusion heat treatment model (a homogeneous diffusionheat treatment step),

a preliminary rolling model (a preliminary rolling step),

a heating model (a reheating step),

a hot rolling model (a rolling step),

a transformation model (a cooling step),

a quenching model (a quenching step),

a tempering model (a tempering step) and

a normalizing step (a normalizing step);

a precipitation model for completing a computation for the state ofsolid solution and precipitation of elements in each step; and

a structure-property model for completing a computation for themechanical properties of the steel product based on the results of thecomputation.

In any model, the calculation for the metallurgical phenomena in thestep is successively performed so as to complete a computation for thestate of the metallic structure. In order for a model to be accessedaccording to the steps through which the steel product passes, and acomputation is completed to automatically estimate the state of themetallic structure and the properties of the final steel product,software is prepared and loaded into the computer.

The construction and computation of each model will now be described indetail.

In the production of a steel plate, the most fundamental production flowcomprises casting-reheating of slab-rolling-cooling. Accordingly, at theoutset, models corresponding to the fundamental flow will be described.

All the models comprise formulae representing metallurgical phenomena inrespective steps, such as growth of grain, recovery, recrystallizationand transformation. Many studies have hitherto been completed on themethod of describing individual metallurgical phenomena, and theformulae including the following formulae constituting a precipitationmodel and a structure-property model are reported in, for example, ISIJInternational Vol. 32 (1992), No. 3, p. 395.

FIG. 2 is a diagram showing a construction and a computing flow of acasting model.

In this model, information concerning steel ingredients and castingconditions are input. The ingredients are expressed in terms of % byweight and carbon (C), silicon (Si), manganese (Mn), phosphorus (P),sulfur (S), copper (Cu), nickel (Ni), chromium (Cr), molybdenum (Mo),niobium (Nb), vanadium (V), titanium (Ti), tantalum (Ta), aluminum (Al),boron (B), tungsten (W) , cobalt (Co), calcium (Ca), rare earth elements(Rem), nitrogen (N) and oxygen (0), etc. The information concerningcasting conditions includes slab thickness, drawing rate, cooling waterquantity density and elapsed time after drawing in the case of acontinuous casting process, and steel ingot size in the case of an ingotmaking process.

In the computation step, at the outset, the thermal history iscalculated using a casting temperature model based on castingconditions. The calculation for a phase diagram is then performed basedon the information concerning steel ingredients. As is well known in theart, since the crystalline structure varies (transforms) depending upontemperature, the temperature calculated at given elapsed time intervalsis checked against a phase diagram to determine whether or not the γ/αtransformation has been initiated. If the γ/α transformation has notbeen initiated, it is determined whether or not the temperature haslowered to such an extent that the γ-phase forms. If the judgment isnegative, the fractions of the liquid phase, δ phase and γ-phase arecalculated, and with respect to the γ-phase, the fractions of anequi-axed crystal and a columnar crystal are calculated. Further, thedistribution of the alloying elements between phases is calculated todetermine the segregation concentration. This calculation is repeateduntil the temperature necessary for forming the γ-phase is attained.When it is determined that the temperature is below the γ-phaseformation temperature, the grain growth in terms of γ -grain diameterand the change in segregation concentration owing to diffusion of thealloying elements are calculated. On the other hand, when it isdetermined that the temperature has lowered to the γ/α transformationinitiation temperature, the model is transferred to a transformationmodel, which will be described later.

The computation flow shown in FIG. 2 is not limited to the case wherethe computation is performed to the final step, and when the transfer tothe next step has been conducted during the process, that is, whenrolling (straightforward rolling) or insertion into the heating furnace(hot charge rolling) has been effected, the state of the metallicstructure at that time is transferred to a model corresponding to thenext step.

FIG. 3 is a diagram showing a construction and a computing flow of aheating model.

In this model, the results of a computation of a model corresponding tothe previous step (for example, casting) or information concerning aprevious structure arbitrarily set without computation, informationconcerning a slab, such as ingredients and size, and heating conditionsare input. The furnace atmosphere temperature and the period of time inthe furnace or the temperature rise rate and the holdingtemperature/time can be input as heating conditions. It is possible toinput actual conditions as well as virtual heating conditions.

In the computation step, the thermal history is computed based on slabsize and heating conditions using a heating temperature model. Thecalculation for a phase diagram is then performed based on informationconcerning steel ingredients.

As is well known in the art, since the crystal structure of steel varies(transforms) depending upon temperature, the state of the heatedstructure (austenite grain diameter) is computed using differenttechniques for each state. Specifically, if necessary, the temperaturecomputed at given elapsed time intervals is checked against a phasediagram so as to perform a calculation for the growth of austenitegrains in a region of austenite+ferrite+cementite, a region ofaustenite+ferrite and a single region of austenite. At that time, aprecipitation model, which will be described later, is computed parallelwith the calculation for grain growth so as to add the state of solidsolution and precipitation of each element during heating to thecomputing conditions of the heated structure (austenite grain diameter).

Further, in a single phase region of austenite, a change in the state ofsegregation is computed using a diffusion model based on the results ofa computation of the above-described thermal history.

FIG. 4 is a diagram showing a construction and a computing flow of a hotrolling model.

In this model, information concerning a slab, such as ingredients andsize, the results of a computation of the model in the previous step orarbitrarily assumed conditions corresponding to the model and rollingconditions, are input. In the case of straightforward rolling, theprevious step comprises casting, while in other cases, it usuallycomprises the reheating of a slab. The rolling conditions include asteel product thickness on inlet and outlet sides, the period of timebetween passes, the roll diameter and the number of roll revolutions.

In the computation step, at the outset, the thermal history is computedfrom a rolling temperature model based on the input items, andcorresponding strain/equivalent strain rate distribution is computedfrom a strain model. In the temperature calculation, the removal of heatand other parameters are also taken into consideration.

When the steel is rolled in a plurality of passes, since the dislocationdensity varies between passes during the course of rolling→recovery→recrystallization as shown in FIG. 5, a calculation forrecrystallization and recovery is performed for each pass. Calculationsfor the austenite grain diameter, average dislocation density, etc. foreach pass and after completion of rolling are performed as follows. Atthe outset, constant and initial values necessary for the calculationare set, and the intergranular area per unit volume of austenite afterrolling is calculated.

When the draft of the rolling is large, instantaneous recrystallization,that is, dynamic recrystallization, occurs. Accordingly, it isdetermined whether or not dynamic recrystallization has occurred, andwhen the judgment is positive, the dislocation density andrecrystallized grain diameter are calculated. When the dynamicrecrystallization is not completed, the time taken for recrystallizationto occur is then calculated and a calculation of the recovery time andstatic recrystallization (recrystallinity and recrystallized graindiameter) is completed.

On the other hand, when the recrystallization is completed, acalculation of the grain growth is performed and the average graindiameter of the crystal grain and the average dislocation density arefurther computed. This step is repeated until the final pass so as toobtain final pass information (intergranular area of austenite anddislocation density). A precipitation model, which will be describedlater, is computed parallel with the above step so as to add the stateof solid solution and precipitation of each element during rolling tothe above-described computation conditions for a rolled structure.

FIG. 6 is a diagram showing a construction and a computing flow of atransformation model.

In this model, information concerning a steel product or a slab, such asingredients and size, the results of a computation of the model in theprevious model or arbitrarily assumed conditions corresponding to themodel and cooling conditions are input. The previous step includes,besides rolling and reheating, the above-described cooling step duringcasting. The cooling conditions include a classification into aircooling and water cooling, a water quantity density within a coolingdevice and a travel rate of the steel product, etc. The forced coolingis not limited to water cooling, and cooling with a molten salt, whichis a future promising means, can be applied if a physical constant, suchas a heat transfer coefficient, is set.

In the computation step, at the outset, the thermal history duringcooling is computed from a cooling temperature model based on theabove-described input items, and, at the same time, the state of a solidsolution and precipitation during cooling is computed from aprecipitation model.

The transformation behavior of the steel is influenced by the state ofaustenite (the austenite grain diameter or intergranular area per unitvolume, the residual dislocation density and the state of the solidsolution and precipitation of a precipitate) before transformation andthe cooling rate. In the present model, a calculation is performed basedon the above-described input items for the advance of transformation,the fractions of individual structures such as intergranular ferrite,transgranular ferrite, pearlite, bainite and martensite and further thegrain diameter and fraction when the ferrite is granular.

The calculation method is as follows.

At the outset, a calculation for a phase diagram of the ingredients isperformed, and conditions (temperature region) under which individualstructures are thermodynamically formed are determined. Then, withrespect to a structure that has been judged to be formable, an incrementof the degree of transformation within a given small period of time isdetermined, while with respect to ferrite, an increment of the number offormed grains in this period of time is determined.

Further, when ferrite is formed, it is determined whether the shape isacicular or granular. When the shape is granular, the number of formedgrains and the increment of the degree of transformation are regardedrespectively as the increment of the number of ferrite grains and theincrement of the amount of granular ferrite. On the other hand, when theshape is acicular, only the increment of the degree of transformation isdetermined. A temperature change according to the degree oftransformation is then calculated for the purpose of conducting afeedback of the generation of heat accompanying the transformation ascooling temperature information. Temperature changes are also fed backto the parallel precipitation model for use in the computation for thestate of solid solution and precipitation during cooling.

The above-described calculation is repeated until the cooling(transformation) is completed, and the fractions of respective finalstructures and the grain diameter of granular ferrite are determinedfrom the fraction and the number of grains by adding the increment ofthe degree of transformation and the increment of the number of granularferrite grains. Further, the average temperature, at which each offerrite, pearlite, bainite, martensite, etc. is formed, (averageformation temperature), is calculated based on the degree oftransformation and the temperature change corresponding to the degree oftransformation.

In the above-described calculation, the ferrite is classified intogranular ferrite and acicular ferrite for the purpose of estimating theproperties of the steel product with a high degree of accuracy. Thisclassification derives from the fact that shapes, such as granular andacicular shapes, participate in the properties of the steel product. Theinformation concerning the average formation temperature is necessarybecause the properties of the steel product varies depending upon theformation temperature. It is used in the structure/property model etc.which will be described later.

The homogeneous diffusion heat treatment model, preliminary rollingmodel, quenching model, tempering model and normalizing model will nowbe described. These models each comprise a combination of theabove-described heating model, hot rolling model, transformation model,etc.

The homogeneous diffusion heat treatment model comprises a heating modeland a transformation model.

In this model, initial conditions, i.e., information concerning steelingredients, slab size, information about a metallic structure,information about segregation, information about solid solution andprecipitation, and further heating and cooling conditions for thehomogeneous diffusion heat treatment such as the heating furnaceatmosphere temperature, the period of time in the furnace and theclassification of cooling after withdrawal from the furnace are input.The results of the computation for the casting model, which is a generalprevious step, constitute the initial condition. It is also possible toprovide arbitrarily assumed initial conditions, which enables thecomputation time to be shortened.

The preliminary rolling model comprises the heating model, hot rollingmodel and transformation model.

The initial condition comprises the results of a computation of theprevious step, for example, a model corresponding to a casting orhomogeneous diffusion heat treatment or an arbitrarily assumed state ofa metallic structure corresponding thereto. In this model, theabove-described initial conditions, information concerning solidsolution and precipitation, reheating conditions in the preliminaryrolling, rolling conditions and cooling conditions are input.

The quenching model comprises the above-described heating model andtransformation model, which apply identical computation means.

The initial condition is the state of the metallic structure passingthrough a rolling-cooling step, and comprises the results of acomputation of a model or an arbitrarily assumed state. In this model,the above-described initial conditions and information concerning thesteel product, such as ingredients and size, information concerningsolid solution and precipitation and the conditions for reheating andcooling in the quenching step are input.

The tempering model consists of the above-described heating model alone,and the temperature region is limited to Acl or less.

The initial condition is the state of a metallic structure passedthrough a rolling-cooling step or a rolling-cooling step and a quenchingstep and comprises the results of a computation of a model or anarbitrarily assumed state. In this model, the above-described initialconditions and information concerning a steel product, such asingredients and size, information concerning a solid solution andprecipitation and the reheating and cooling conditions in the temperingstep are input.

A precipitation model as well is parallel computed according to thecomputed thermal history based on the above-described input items forconducting a computation for the decomposition/formation of a metallicstructure and the state of a carbide/precipitate.

The normalizing model comprises the above-described heating model andtransformation model and is fundamentally applied using by the samecomputation means as that of the quenching model.

The initial condition is the state of a metallic structure passedthrough the rolling-cooling step, and the state of a metallic structurecomprises the results of a computation of a model or an arbitrarilyassumed state. In this model, the above-described initial conditions,information concerning ingredients and size and the reheating andcooling conditions in the normalizing step are input.

In the precipitation model wherein the state of the solid solution andprecipitation of elements throughout the above-described individualsteps is computed, the state of the solid solution and precipitation ofeach element is computed from various information (including the stateof solid solution and precipitation) in the step previous to an intendedstep and the conditions for the intended step. This precipitation modelis always computed parallel to each above-described model andsuccessively utilized in the computation of each model, and the results(information about a metallic structure) are fed back to theprecipitation model.

FIG. 7 is a diagram showing a construction and a computing flow of astructure-property model.

The object of the structure-property model is to compute the yieldstrength (YP), tensile strength (TS) and ductile/brittle fracturetransition temperature (vTrs) in the V Charpy impact test using acombination of the above-described individual models corresponding tothe steel product production steps. In this model, the results of acomputation of a model corresponding to the final step, through whichthe steel product has passed, are input.

At the outset, a calculation for the hardness of individual structures(ferrite, pearlite, bainite, martensite, etc. ) and a calculation forthe yield strength are conducted based on the above-described inputitems. The tensile strength is then calculated using the calculatedhardness value. Further, the ductile/brittle fracture transitiontemperature in the V Charpy impact test is calculated to finish theprocessing.

As with all of the above-described models, the present model can besolely computed. In this case, the ferrite grain diameter and measuredvalues or arbitrarily assumed values of the fraction/hardness of eachstructure may be input to compute the properties of the steel product.

When the above-described series of computations are conducted, itbecomes possible to automatically estimate the properties of the steelproduct. The results are stored in a recording medium such as a floppydisk and printed by means of a printer.

The divided time intervals, the number of steel sheet divisions in thesheet direction, etc. for conducting a computation for informationconcerning the structure and the solid solution and precipitation can beset according to the purpose, which contributes to an improvement incomputation accuracy and shortening of computing speed and enablesinformation concerning the structure and properties deviation in thesheet thickness direction to be obtained.

TEST EXAMPLE

Table 1 shows chemical ingredients of a steel applied to the presentinvention, Table 2 shows production steps, and FIGS. 8 to 10 each show acomparison of measured values of yield strength (YP), tensile strength(TS) and the ductile/brittle fracture transition temperature (vTrs) in aV Charpy impact test with calculated values determined by theabove-described estimation method.

As is apparent from FIGS. 8 to 10, the measured values are close to thecalculated values, which substantiates that the estimation could be madewith a very high degree of accuracy. Further, in Table 2, in the case ofsteels A-6, A-7 and A-8, a calculation wherein the cast structure isassumed without a computation for the cast model is also performed. Theresults are indicated by a black dot ( ) in FIGS. 8 to 10. In this caseas well, the estimation accuracy is comparable to that of theabove-described case and on a level satisfactory for practical use. Whenthe cast model is not computed, the total computation time can beshortened and in the present test example, a shortening of about 25% canbe attained.

Thus, since a highly reliable estimation is possible, it is alsopossible to perform a computation for production conditions of a productaccording to the properties required by users.

                                      TABLE 1                                     __________________________________________________________________________    (wt. %, *: ppm)                                                               __________________________________________________________________________    Steel                                                                            C  Si Mn P  S  Al Cu Ni Cr Mo Nb                                           __________________________________________________________________________    A  0.07                                                                             0.17                                                                             0.91                                                                             0.011                                                                            0.004                                                                            0.024                                                                            0.22                                                                             0.15  0.54                                            B  0.13                                                                             0.25                                                                             1.24                                                                             0.016                                                                            0.004                                                                            0.022    0.13  0.021                                        C  0.05                                                                             0.28                                                                             1.40                                                                             0.007                                                                            0.001                                                                            0.020                                                                            0.04     0.23                                                                             0.008                                        __________________________________________________________________________    Steel                                                                            V   Ti Ta B   W   Co  Ca  Rem N*                                                                              O*                                         __________________________________________________________________________    A  0.035                                                                             0.011 0.0005      0.0021                                                                            0.0007                                                                            26                                                                              30                                         B                        0.0035  33                                                                              31                                         C      0.013                                                                            0.005  0.0006                                                                            0.012       28                                                                              26                                         __________________________________________________________________________

                  TABLE 2                                                         ______________________________________                                             Cast-              Reheat-                                                                              Roll-                                                                              Cool-                                     Steel                                                                              ing    SP.sup.1)                                                                            BD.sup.2)                                                                          ing    ing  ing   Q.sup.3)                                                                          T.sup.4)                                                                          N.sup.5)                    ______________________________________                                        A-1  ◯             ◯                                                                      ◯                             A-2  ◯             ◯                                                                      ◯                                                                           ◯                   A-3  ◯             ◯                                                                      ◯                                                                       ◯                                                                     ◯                   A-4  ◯             ◯                                                                      ◯ ◯               A-5  ◯      ◯                                                                        ◯                                                                      ◯                             A-6   ◯*    ◯                                                                        ◯                                                                      ◯                                                                           ◯                   A-7   ◯*    ◯                                                                        ◯                                                                      ◯                                                                       ◯                                                                     ◯                   A-8   ◯*    ◯                                                                        ◯                                                                      ◯ ◯               B-1  ◯                                                                        ◯                                                                             ◯                                                                        ◯                                                                      ◯                             B-2  ◯                                                                        ◯                                                                             ◯                                                                        ◯                                                                      ◯                                                                           ◯                   B-3  ◯                                                                        ◯                                                                             ◯                                                                        ◯                                                                      ◯                                                                       ◯                                                                     ◯                   B-4  ◯                                                                        ◯                                                                             ◯                                                                        ◯                                                                      ◯ ◯               C-1  ◯                                                                        ◯                                                                        ◯                                                                      ◯                                                                        ◯                                                                      ◯                             C-2  ◯                                                                        ◯                                                                        ◯                                                                      ◯                                                                        ◯                                                                      ◯                                                                           ◯                   C-3  ◯                                                                        ◯                                                                        ◯                                                                      ◯                                                                        ◯                                                                      ◯                                                                       ◯                                                                     ◯                   C-4  ◯                                                                        ◯                                                                        ◯                                                                      ◯                                                                        ◯                                                                      ◯ ◯               C-5  ◯ ◯                                                                      ◯                                                                        ◯                                                                      ◯                             C-6  ◯ ◯                                                                      ◯                                                                        ◯                                                                      ◯                                                                           ◯                   C-7  ◯ ◯                                                                      ◯                                                                        ◯                                                                      ◯                                                                       ◯                                                                     ◯                   C-8  ◯ ◯                                                                      ◯                                                                        ◯                                                                      ◯ ◯               ______________________________________                                         Note:                                                                         .sup.1) homogeneous diffusion heat treatment, .sup.2) preliminary rolling     .sup.3) quenching, .sup.4) tempering, .sup.5) normalizing,                    *Integrated calculation was also practiced, assuming the casting              structure.                                                               

EFFECT OF THE INVENTION

The present invention has the following effects because in each step ofcasting for steel making (a continuous casting process or a steel ingotprocess), a treatment (a homogeneous diffusion treatment or preliminaryrolling) of a slab (a steel ingot), reheating of a slab (a steel ingot),rolling, cooling (air cooling and forced cooling), a heat treatment(quenching, tempering or normalizing), which are steps for producing ahot rolled steel product, an index for a steel product property in eachstep is computed from the conditions in the step, steel ingredients,size, etc., and the properties of the steel product are estimated frominformation concerning the metallic structure, information concerningthe solid solution and precipitation (amount and size), etc. in thefinal step.

(1) As opposed to the prior art, testing and measuring a finishedproduct can be significantly simplified or rendered unnecessary.

(2) Production conditions capable of satisfying quality requirements canbe set at the stage of production.

(3) It has become possible to conduct the estimation and control of theproperties of the steel product wherein the properties are estimated ineach step during the production and the process conditions arecontrolled so that the quality requirements are satisfied.

(4) The properties of the steel product can be estimated by providingconditions assumed in the development of a novel steel product and anovel process, which significantly reduces the burden and developmenttime of the novel steel product and process.

UTILIZATION IN INDUSTRY

As described above, the method of estimating the properties of a steelproduct according to the present invention can be widely applied to asteel plate, a hot coil, a shape steel, etc. in the steel industry.Further, the use of a method for estimating the properties of a steelproduct according to the present invention can facilitate properties andprocess control and the design of ingredients with a high degree ofaccuracy.

We claim:
 1. A method of estimating, based on steel ingredients andproduction conditions, the properties of a steel product produced bysubjecting a slab cast according to a continuous casting process or aningot making process to at least rolling and cooling, comprising thesteps of:computing the temperature of a slab based on casting conditionsincluding at least slab size, drawing rate and cooling water quantitydensity and time wherein said steel slab has a metallic structure aftercasting including an equi-axed crystal and columnar crystal fraction,and equi-axed and columnar austenite grains having a diameter, andfurther completing a computation for the state of the metallic structureafter casting including at least the fraction of equi-axed crystal andcolumnar crystal, the diameter of equi-axed and columnar austenitegrains, the state of solid solution and precipitation and the state andconcentration of segregation; completing a computation for determiningthe state of the metallic structure after rolling including at least thediameter of an austenite grain, the intergranular area of austenite perunit volume, the dislocation density within austenite and the state ofsolid solution and precipitation based on the state of the metallicstructure before the rolling and the temperature of the steel productcomputed from rolling conditions including at least the size of thesteel product on input and output sides in each pass anti the period oftime between passes; completing a computation for determining the stateof the metallic structure after cooling including at least the fractionof each structure of ferrite, pearlite, bainite and martensite, thegrain diameter of ferrite and the state of solid solution andprecipitation based on the state of the metallic structure beforecooling and the temperature of the steel product computed from coolingconditions including at least classification into water cooling and aircooling and the water quantity density and travel rate within a device;and when the steel product has a final state metallic structure,completing a computation for determining the properties of the steelproduct based on the final state of the metallic structure.
 2. Themethod according to claim 1, which further comprises, in order to copewith optional reheating conducted prior to rolling, said reheatingoccurring in a furnace having an atmosphere, said atmosphere having atemperature,the step of completing a computation for determining thestate of the metallic structure after reheating including at least thegrain diameter of austenite and the state of solid solution andprecipitation based on the state of the metallic structure beforereheating and the temperature of the slab computed from reheatingconditions, including at least the furnace atmosphere temperature andthe period of time in the furnace.
 3. The method according to claim 2,which further comprises, in order to cope with a homogeneous diffusionheat treatment conducted optionally subsequent to casting,the step ofcompleting a computation for determining the state of the metallicstructure after the homogeneous diffusion heat treatment including atleast the state of solid solution and precipitation of each element andthe state and concentration of segregation based on the state of themetallic structure after casting and homogeneous diffusion treatmentconditions including at least the furnace atmosphere temperature, theperiod of time in the furnace and cooling conditions after withdrawalfrom the furnace.
 4. The method according to claim 3, which furthercomprises, in order to cope with preliminary rolling conductedoptionally subsequent to the homogeneous diffusion heat treatment,thestep of completing a computation for determining the state of themetallic structure after the preliminary rolling including at least thestate of solid solution and precipitation of each element and the stateand concentration of segregation by using, as all initial condition, thestate of the metallic structure after the homogeneous diffusion heattreatment, said step comprising the substeps of: completing acomputation for determining the state of the metallic structure afterreheating in the preliminary rolling including at least the graindiameter of austenite and the state of solid solution and precipitationof each element based on the state of the metallic structure before thereheating in the preliminary rolling and the slab temperature computedfrom reheating conditions including the furnace atmosphere temperatureand the period of time in the furnace; completing a computation fordetermining the state of the metallic structure after the rolling in thepreliminary rolling including at least the grain diameter of austenite,the dislocation density within austenite and the state of solid solutionand precipitation based on the state of the metallic structure beforethe rolling in the preliminary rolling and the temperature of the steelproduct computed from rolling conditions including at least the size ofthe steel product on inlet and outlet sides in each pass and the periodof time between passes, and completing a computation for determining thestate of the metallic structure after cooling including at least thefraction of each structure of ferrite, pearlite, bainite and martensite,the grain diameter of ferrite and the state of solid solution andprecipitation based on the state of the metallic structure before thecooling in the preliminary rolling and the temperature of the steelproduct computed from cooling conditions including at least theclassification into water cooling and air cooling and the water quantitydensity and travel rate within a cooling device.
 5. The method accordingto claim 2, which further comprises, in order to cope with thepreliminary rolling conducted optionally subsequent to the casting,thestep of completing a computation for determining the state of themetallic structure after the preliminary rolling including at least thestate of solid solution and precipitation of each element and the stateand concentration of segregation, the fraction of the structure and thegrain diameter of ferrite by using, as an initial condition, the stateof the metallic structure after the casting; said step comprising thesubsteps of: completing a computation for determining the state of themetallic structure after reheating in the preliminary rolling includingat least the grain diameter of austenite and the state of solid solutionand precipitation of each element based on the state of the metallicstructure before the reheating in the preliminary rolling and the slabtemperature computed from reheating conditions including at least thefurnace atmosphere temperature and the period of time in the furnace;completing a computation for determining the state of the metallicstructure after rolling in the preliminary rolling including at leastthe grain diameter of austenite, the dislocation density withinaustenite and the state of solid solution and precipitation based on thestate of the metallic structure before the rolling in the preliminaryrolling and the temperature of the steel product computed from rollingconditions including at least the size of the steel product on inlet andoutlet sides in each pass and the period of time between passes, andcompleting a computation for determining the state of the metallicstructure after cooling in the preliminary rolling including at leastthe fraction of each structure of ferrite, pearlite, bainite andmartensite, the grain diameter of ferrite and the state of solidsolution and precipitation based on the state of the metallic structurebefore cooling in the preliminary rolling and the temperature of thesteel product computed from cooling conditions including at leastclassification into water cooling and air cooling and the water quantitydensity and travel rate within a cooling device.
 6. The method accordingto claim 1 which further comprises, in order to cope with temperingconducted optionally subsequent to the cooling,the step of completing acomputation for determining the state of the metallic structure aftertempering including at least the fraction of each structure of ferrite,pearlite, bainite and martensite, the grain diameter of ferrite and theamount and size of a precipitate and the state of solid solution basedon the state of the metallic structure before the tempering andtempering conditions including at least heating and cooling conditions.7. The method according to claim 6, which further comprises, in order tocope with quenching conducted optionally prior to the tempering,the stepof completing a computation for determining the state of the metallicstructure after quenching including at least the state of solid solutionand precipitation, the fraction of each structure of ferrite, pearlite,bainite and martensite and the grain diameter of ferrite based on thestate of the metallic structure after the cooling and quenchingconditions including at least heating and cooling conditions.
 8. Themethod according to claim 1 which further comprises, in order to copewith normalizing conducted optionally subsequent to the cooling,the stepof completing a computation for determining the state of the metallicstructure after normalizing including at least the state of solidsolution and precipitation, the fraction of each structure of ferrite,pearlite, bainite and martensite and the grain diameter of ferrite basedon the state of the metallic structure after the cooling and normalizingconditions including at least heating and cooling conditions.
 9. Amethod of estimating, based on steel ingredients and productionconditions, fine quality of a steel product produced by subjecting aslab cast according to a continuous casting process or an ingot makingprocess to at least reheating, rolling, cooling and tempering, the steelhaving a metallic structure state after each processing step, comprisingthe steps of:completing a computation for determining the state of themetallic structure after rolling including at least the grain diameterof austenite, the intergranular area of austenite per unit volume, thedislocation density in austenite and the state of solid solution andprecipitation based on the state of the metallic structure before therolling and the temperature of the steel product computed from rollingconditions including at least the size of the steel product on inlet andoutlet sides in each pass and the period of time between passes;completing a computation for determining the state of the metallicstructure after cooling including at least the fraction of eachstructure of ferrite, pearlite, bainite and martensite, the graindiameter of ferrite and the state of solid solution and precipitationbased on the state of the metallic structure before the cooling and thetemperature of the steel product computed from cooling conditionsincluding at least classification into water cooling and air cooling andthe water quantity density and the travel rate within device; completinga computation for determining the state of the metallic structure aftertempering including at least the fraction of each structure of ferrite,pearlite, bainite and martensite based on the state of the metallicstructure before the tempering and tempering conditions including atleast heating and cooling conditions; and when the steel product has afinal metallic structure state, completing a computation for determiningthe properties of the steel product based on the final state of themetallic structure.
 10. The method according to claim 9, which furthercomprises, in order to cope with quenching conducted optionally prior tothe tempering,completing a computation for determining the state of themetallic structure after quenching including at least the state of solidsolution and precipitation, the fraction of each structure of ferrite,pearlite, bainite and martensite and the grain diameter of ferrite basedon the state of the metallic structure after cooling and the quenchingconditions including at least heating and cooling conditions.
 11. Amethod of estimating, based on steel ingredients and productionconditions, the properties of a steel product produced by subjecting aslab cast according to a continuous casting princess or an ingot makingprocess to reheating, rolling, cooling and normalizing, comprising thesteps of:completing a computation for determining the state of ametallic structure after rolling including at least the grain diameterof austenite, the intergranular area of austenite per unit volume, thedislocation density in austenite and the state of solid solution andprecipitation based on the state of the metallic structure before therolling and the temperature of the steel product computed from rollingconditions including at least the size of the steel product on inlet andoutlet sides in each pass and the period of time between passes;completing a computation for determining the state of the metallicstructure after cooling including at least the fraction of eachstructure of ferrite, pearlite, bainite and martensite, the graindiameter of ferrite and the state of solid solution and precipitationbased on the state of the metallic structure before the cooling and thetemperature of the steel computed from cooling conditions including atleast classification into water cooling and air cooling and the waterquantity density and the travel rate within a cooling device; completinga computation for determining the state of the metallic structure afternormalizing including at least the state of solid solution andprecipitation, the fraction of each structure of ferrite, pearlite,bainite and martensite based on the state of the metallic structureafter cooling and the normalizing conditions including at least heatingand cooling conditions; and when the steel product has a final metallicstructure state, completing a computation for determining the propertiesof the steel product based on the final state of the metallic structure.